If you look at a standard TJ frame...it is pretty bereft of cross-members. There are really only three welded to the frame rails (front tube, rear bumper cross-member, and the rear shockmount. They bolted the transmission skid in as another crossmember. The way the torque boxes are designed on the tub makes me think that they did intend for the tub to help "unitize" the frame a bit.
I read a jalopnik article recently talking about the difference between a traditional body on frame pickup truck, like the Tacoma or Colorado (and the full-size versions) and a uni-body "truck" like the Honda Ridgeline. I think you'd enjoy it...
https://jalopnik.com/mid-size-trucks-dont-need-frames-1785674405
Anyway, in short, the design considerations of the ladder frame vs the unibody are interesting. The sail panels on the Avalanche (and first gen Ridgeline) are there to stiffen the body so frame twisting doesn't rip the body apart. If you think about a traditional Pick-up, the bed is seperate from the cab. That means the frame can twist and move a bit, and the two body sections are free to move independently of one another.
Bringing this back around to the TJ...without doorframes and no real B-pillar to speak of (and a folding A pillar), I'd bet the tub would tear itself apart in short order if you were to run without the roll bar bolted in (and the upper spreaders connected to the windshield). Everything is a system...designed to work together. Start changing it and you better be thinking about EVERYTHING that its connected to. Pretty interesting discussion to follow on the FB TJ-6 page when
@mrblaine posted his question about extending the torque boxes for his TJ-6 project.